#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my $foo = 1;
my $bar = 'baz';
my $baz = '';

my %foohash = (
    $foo ? ( foo => $foo ) : (),
    bar => $bar,
    $baz ? ( baz => $baz ) : (),
    $ARGV[0] ? ( version => 'snmpv' . $ARGV[0] ) : ()
);

print Dumper(\%foohash);

exit;

__END__

Output:
$VAR1 = {
          'bar' => 'baz',
          'foo' => 1
        };